Flexor Digitorum Accessorius Longus
Leg & Ankle
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Flexor digitorum accessorius longus is an uncommon extra muscle that can travel through the tarsal tunnel. Because that tunnel is already crowded with nerves, vessels, and tendons, this muscle can contribute to tibial nerve compression and tarsal tunnel syndrome.
Function
Assists flexion of toes 2–5 by reinforcing the pull of flexor digitorum longus. Depending on its attachment it may also assist quadratus plantae and contribute slightly to plantarflexion and stabilization of the foot during gait.
Origin
Highly variable. Reported origins include the posterior or medial tibia, posterior fibula, deep fascia of the leg, intermuscular septa, soleus, flexor hallucis longus, or nearby connective tissues.
Insertion
Most commonly joins the tendon of flexor digitorum longus or quadratus plantae. Less commonly it may attach to the calcaneus or other structures within the plantar foot.
Innervation
Tibial nerve, typically through muscular branches supplying the deep posterior compartment.